The majority of that margin doesn't have you profiting, and it's speculation.

Some coops have flip or sale tax which if you ignore or miss can cut deep into your profit.2.

If you are able to roll over your employer 401k and IRA's into a solo401k and/or self-directed IRA, you can invest in real estate ventures with those accounts, but you cannot benefit or profit from those properties in the form of personal use or personal income.

For example, if I were to sell a property and net $50,000 profit, could I elect to exchange $40K of the proceeds towards a new purchase, pay short-term cap gains on $10K and keep the balance? 

I have seen many properties where investors squeeze as much profit as possible without doing any maintenance then when it's at a breaking point they sell it to a new investor because it just looks so great on paper.If you do end up putting an offer on such property I would make sure to do a very good due diligence and even talking to the current renters.

With the interest on the remaining balance, the seller actually profits way more than their initial goal price. 

A partner typically shares in the profits (or losses) on a deal.

Looks like you learned a lot and made a good profit!  

My idea is to split lot, and partner with a builder to build a home and sell it, splitting the profit.

Let's say I sell the property at the ARV that the cashout refi is based-on, and I don't profit from the sale.